The efficiency is measure of the effectiveness of the radiator in achieving a uniform temperature across its surface. A typical efficiency is >80%. The efficiency depends among other factors on the in-plane heat conductivity of the radiator. An acceptable efficiency may be realized through the use of (linear) heat transporting devices such as heat pipes, loop heat pipes, or mechanically pumped fluid loops. These devices are embedded in or attached to a panel manufactured from a material with a high ratio of heat conductivity to weight, such as aluminium.
For typical telecommunications satellites, a radiator generally consists of an (aluminium) sandwich panel with an (aluminium) honeycomb core with embedded constant conductance heat pipes spaced at regular intervals. Besides acting as radiator, such a panel also has a structural function in the sense that it forms (part of) the spacecraft load carrying structure. The panels are found typically on the North- and South walls of telecommunications spacecraft, and are usually carried out as sandwich panels with a honeycomb core equipped with embedded heat pipes for transport of heat.
The advantage of these devices is that the thermal function can be fulfilled at a minimal cost of mass and without adding any additional parts to the spacecraft. However such devices are required to fulfill a specific structural function, placing requirements on the device that may be incompatible with the thermal requirements or simply inconvenient in some circumstances. For instance, these structures by definition are required to be stiff whereas it may be desirable to provide a flexible radiator for certain reasons.
Further radiators are known which typically consist of a rigid panel manufactured from highly conductive material such as aluminium, high-conductive carbon fiber and in some cases copper. In some cases the panel is a composite structure with a honeycomb core for mass efficiency. These devices are mounted to a supporting structure with standoffs which are often made from a low thermally conductive material. They are insulated from the supporting structure by a multilayer insulation blanket.
The advantage of such a device is that it can be optimized for its thermal function without fulfilling a mechanical function and as such they are employed in cases where radiator temperatures are incompatible with the temperature requirements of the supporting structure. This is typically the case in devices providing cooling to infrared instrument systems and in East-West radiator systems on telecommunications satellites whereby the radiator may achieve very high temperatures when exposed to the sunlight.
As spacecraft payloads increase also the heat generated by the payload increases, creating demand for ever larger radiators. In this connection, deployable radiators have been developed. These deployable radiators consist of one or more rigid panels interconnected by hinges and fitted with a flexible heat transportation device such as a loop heat pipe or mechanically pumped fluid loop. The advantage of such a device is that a large radiating surface can be launched into orbit while being stored during launch in a very limited volume. The rigidity of the panels themselves however imposes limitations on the use thereof.
Thus, in a number of cases it is not possible or desirable to combine a structural function with the thermal function of a radiator panel. As mentioned before, this is for instance the case when the required radiator temperature is incompatible with the structure temperature. As an example, reference is made to radiators which provide thermal control to payloads with a temperature significantly higher or lower than room temperature.
Furthermore, the use of a thermal switch (e.g. a variable conductance heat pipe or loop heat pipe with bypass valve) may cause temperatures experienced by the radiator to be incompatible with the structure's temperature limits. As an example, reference is made to a radiator providing thermal management to a room temperature unit. Such radiator may nevertheless experience very high temperatures when the thermal switch is open and the radiator is exposed to direct sunlight.
Also, it may occur that the structural function places requirements on the panel which are incompatible with the thermal function. For instance, the position of attachment provisions or inserts may interfere with heat pipe layout. Problems may also rise in case strength and/or stiffness requirements imply the choice of an unsuitable material such as titanium, steel.
Apart from these circumstances, it may be beneficial to separate the structural and thermal functions into two different systems for reasons of mass, cost or manufacturability, etc. Typically in such cases a dedicated radiator panel is mounted on the structure and thermally decoupled by the use of standoffs and multilayered insulation (MLI).